1. Field of the Invention
Embodiments of this invention relate to sensorless rotor position determination in reluctance machines, particularly, but not exclusively, in switched reluctance machines.
2. Description of Related Art
The control and operation of switched reluctance machines generally are described in the paper “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives” by J M Stephenson and R J Blake delivered at the PCIM'93 Conference and Exhibition held in Nurnberg, Germany, 21–24 Jun. 1993, which is incorporated herein by reference. In that paper the “chopping” and “single-pulse” modes of energization of switched reluctance machines are described for operation of the machine at low and high speeds, respectively.
A typical prior art drive is shown schematically in FIG. 1. This includes a DC power supply 11 that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across phase windings 16 of the motor 12 connected to a load 19 by a power converter 13 under the control of the electronic control unit 14. One of the many known converter topologies is shown in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from, or returned to, the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. A resistor 28 is connected in series with the lower switch 22 to provide a current feedback signal. A multiphase system typically uses several “phase legs” of FIG. 2 connected in parallel to energize the phases of the electrical machine.
The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a transducer 15, shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which co-operates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization. This system is simple and works well in many applications. However, the rotor position transducer increases the overall cost of assembly, adds extra electrical connections to the machine and is, therefore, a potential source of unreliability.
Various methods for dispensing with the rotor position transducer have been proposed. Several of these are reviewed in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors” by W F Ray and I H Al-Bahadly, published in the Proceedings of The European Power Electronics Conference, Brighton, UK, 13–16 Sep. 1993, Vol. 6, pp 7–13, incorporated herein by reference.
Many of these methods proposed for routine rotor position estimation in an electrically driven machine use the measurement of phase flux-linkage (i.e. the integral of applied voltage with respect to time) and current in one or more phases. Position is calculated using knowledge of the variation in inductance of the machine as a function of angle and current. This characteristic can be stored as a flux-linkage/angle/current table and is depicted graphically in FIG. 3. The storage of this data involves the use of a large memory array and/or additional system overheads for interpolation of data between stored points.
Some methods make use of this data at low speeds where “chopping” current control is the dominant control strategy for varying the developed torque. Chopping control is illustrated graphically in FIG. 4(a) in which the current and inductance waveforms are shown over a phase inductance period. (Note that the variation of inductance is depicted in idealized form.) These methods usually employ diagnostic energization pulses in non-torque-productive phases (i.e. those phases which are not energized directly from the power supply at a particular moment). A method suited to low-speed operation is that proposed by N M Mvungi and J M Stephenson in “Accurate Sensorless Rotor Position Detection in an S R Motor”, published in Proceedings of the European Power Electronics Conference, Firenze, Italy, 1991, Vol. 1, pp 390–393, incorporated herein by reference. These methods work best at relatively low speeds, where the length of time taken up by a diagnostic pulse is small compared to the overall cycle time of an inductance period. As speed rises, the pulse occupies a longer part of the cycle and soon the point is reached where reliable position information is not available.
Other methods operate in the “single-pulse” mode of energization at higher speeds. This mode is illustrated in FIG. 4(b) in which the current and inductance waveforms are shown over a phase inductance period. These methods monitor the operating voltages and currents of an active phase without interfering with normal operation. A typical higher speed method is described in International Patent Application WO 91/02401, incorporated herein by reference.
Instead of opening both switches simultaneously, there are circumstances in which it is advantageous to open the second switch an angle θf later than θon, allowing the current to circulate around the loop formed by the closed switch, the phase winding and a diode. A typical waveform is illustrated in FIG. 4(c). This technique is known as “freewheeling” and is used for various reasons, including peak current limitation and acoustic noise reduction. Having to store a two-dimensional array of machine data in order to operate without a position sensor is an obvious disadvantage. Alternative methods have been proposed, which avoid the need for the majority of angularly referenced information and instead store data at one angle only. One such method is described in European Patent Application EP0573198A (Ray), incorporated herein by reference. This method aims to determine the phase flux-linkage and current at a predefined angle by adjusting the diagnostic point in accordance with the calculated deviation away from the desired point. Flux-linkage is estimated by integrating (with respect to time) the measurement of the voltage applied to the phase. Two one-dimensional tables are stored in one embodiment, one of flux-linkage versus current at a referenced rotor angle and another of the differential of flux-linkage with respect to rotor angle versus current. By monitoring phase voltage and current, the deviation away from a predicted angle can be assessed, with the aid of the look-up tables, and system operation can be adjusted accordingly.
To avoid the flux-linkage integrator drifting (due to unwanted noise in the system and imperfections in the integrator) it is set to zero at the end of each conduction cycle, when the current has fallen to zero and the phase winding is no longer linking any flux. This method is a “predictor/corrector” method, in that it initially predicts when the rotor will be at a reference position, measures parameters of the machine when it believes the reference position has been reached, and uses the results of these measurements to detect error in the prediction and hence take corrective action by adopting a new prediction for the next reference position.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase, for example between maxima when the rotor poles and the relevant respective stator poles are fully aligned. FIG. 4(a) shows the inductance profile in idealized form, whereas in practice the corners of the profile are rounded due to flux fringing in the air and to saturation of the ferromagnetic paths.
It is known that the shape of the phase current waveform of a switched reluctance machine in single-pulse mode is related to the inductance profile of the phase winding. In particular, the start of the rising portion of the inductance profile, which is due to the onset of overlap between the stator and rotor poles, corresponds to the rollover when the phase current changes from rising to falling in the phase inductance cycle. EP1109309A, incorporated herein by reference, discusses this phenomenon and uses the natural peak in current, in single-pulse operation, as the basis of a rotor position detection method.